This project is designed to answer clinical problems in the management of patients receiving chest physiotherapy (CPT). The objectives are, first, to determine the incidence and clinical significance of hypoxemia during CPT treatments consisting of postural drainage with chest percussion. The second objective is to determine the causes of hypoxemia during CPT. Our methods will allow us to evaluate the contribution to hypoxemia of 1) V/Q abnormality or shunt, 2) hypoventilation, 3) decreased cardiac output, and 4) decreased FI02 due to changes in ventilatory pattern. A third objective is to determine the effectiveness of CPT treatments, objectively, by measuring oxygenation and compliance before, during and after therapy for three hours. A final objective is to investigate the effect of various head-down positions (often used during therapy) on lung volumes and oxygenation in normals and patients with chronic objective pulmonary disease. This aspect of the study is undertaken to provide further understanding of the mechanisms of hypoxemia during CPT. The significance of these studies is that if the occurance of hypoxemia during therapy and the mechanisms causing hypoxemia are better understood, patients particularly at risk of hypoxemia may be identified prior to therapy and actions taken to prevent adverse effects.